In recent years, nanotechnology, often alluded to by use of the term ‘nano’, has attracted a great deal of interest.
Within the realm of nanotechnology, research and development is especially active in nanobiotechnology, a new field combining semiconductor nanotechnology and biotechnology with the potential to achieve fundamental solutions to existing problems.
In this field of nanobiotechnology, particular attention has been focused on biochips, such as DNA chips (or DNA microarrays) and protein chips, as an effective means of for simplifying nucleic acid and protein testing in such areas as clinical diagnosis and drug therapy which is particularly effective in gene analysis. Biochips are substrates formed of glass, silicon, plastic, metal or the like on which multiple differing analytes composed of bio-molecules such as DNA and proteins are placed as spots in high-density arrays (see T. G. Drummond et al.: “Electrochemical DNA sensors,” Nature Biotech. 21, No. 10, 1192-1199 (2003); and J. Wang: “Survey and Summary from DNA biosensors to gene chips,” Nucleic Acids Research 28, No. 16, 3011-3016 (2000)).
In recent years, devices known as MEMS and μTAS, which are manufactured based on technology for evaluating very small targets by combining micromachining technology and microsensing technology, with a functional surface (evaluation part) formed on some portion of a solid substrate by binding functional molecules, or molecules bound to functional molecules onto to the solid substrate, have drawn attention as devices which provide great improvements in evaluation sensitivity and evaluation time. “MEMS” is an abbreviation for Micro Electro Mechanical Systems, and refers to a technology for manufacturing very small devices based on semiconductor technology, or to precision micromachines manufactured using such technology; the term generally refers to systems in which a plurality of functional components—mechanical, optical, fluidic, etc.—have been integrated and miniaturized. “μTAS” is an abbreviation for Micro Total Analysis System, and refers to a chemical analysis system created by miniaturizing, arraying and integrating micropumps, microvalves, sensors and the like. These devices generally have a functional surface composed of functional molecules with specific functions, or molecules bound to such functional molecules, often fixed (bound) in a self-assembling manner on a substrate. Many methods for electrically or optically evaluating reactions at the functional surface are used in these devices.
Among these methods, optical evaluation methods are methods in which a target to be evaluated is modified with an optical label such as a fluorescent dye, and the target is quantitatively evaluated based on the optical intensity. Owing to their high sensitivity, such methods are widely used in DNA chips.
However, because a procedure that involves modifying the target with a label is essential to such methods, cumbersome steps such as labeling and rinsing are required. Other problems include mis-detection due to contamination by the unattached (unreacted) label, and the evaluation of targets which attach non-specifically to the evaluation part rather than binding specifically with the probe.
Accordingly, there exists a desire for the development of highly selective, low-noise methods of evaluation which do not require the target to be modified with a label (non-label techniques) and which avoid the mis-detection of non-specifically adsorbed target.
One known technique for evaluating label-free target molecules is a method in which a marker is modified into a charged probe molecule, the probe molecule is fixed to an electrode and driven by an electrical field, and the drive state is monitored by signals from the marker. When a target molecule binds specifically with the probe molecule, the drive state of the probe changes, which change is evaluated by the marker that has been modified into a probe (see U. Rant et al.: “Dynamic electrical switching of DNA layers on a metal surface,” Nano Lett. 4, No. 12, 2441-2445 (2004); claims of Japanese Patent Application No. 2004-238696; claims of Published U.S. Patent Application No. 2005/069932). The principle underlying this technique is to monitor changes in the signals from the marker that arise due to the changes in the distance between the marker attached to the end of the probe molecule and the substrate as the electrically charged probe molecule is attracted to or repelled by the electric field. So long as the driving frequency is in a frequency range (up to about 1 MHz) that allows formation of an electric double layer as the source of the electric field, evaluation of the target molecule is possible by monitoring signals from the marker that are synchronous with the driving potential.